Carbonation of liquids, particularly for beverages, has taken place for many years. Control of the degree of dissolution of carbon dioxide and other gases in liquids has led to a great deal of experimentation. In some instances, nitrogen has been used in the production and packaging of beers and other beverages primarily to exclude oxygen from the feed water and from contact with the final brewed or bottled product. In addition, it has been found desirable to use nitrogen in a dissolved state in alcoholic beverages, particularly beers, so as to influence the presentation of the beer when the beer is dispensed into the glass or mug.
Depending on the type of beer the carbonation varies, for instance, for a lager beer generally the carbonation level is about 2.5 volumes of carbon dioxide per volume of liquid, and for the dark stout beers that level is about 1.0. Many customers, particularly in Europe express a preference for a tight long-lasting head on dispensed beer. In spite of the presence of various long chain molecules in beers, which molecules have surfactant properties, the desired presentation of a tight long lasting head cannot be achieved with only carbon dioxide in solution. This is true because carbon dioxide is able to permeate rapidly through the thin walls of the initially formed bubbles on the surface of a dispensed beer and hence is lost to the atmosphere which contains a low concentration of carbon dioxide.
It would seem that because the carbon dioxide is supersaturated in the beer that the potential reserve of additional carbon dioxide to replace lost gas would be available. However, this is not normally true because the beer is cold and because modern glass washing methods do not create surface scratches and/or leave deposits which will nucleate carbon dioxide from solution after the beer has come to rest in the glass.
It is known that dissolving a quantity of a weakly soluble gas, conventionally nitrogen, in beer prior to dispense provides high quality presentation in the form of a stable white foam head. Because of its low solubility nitrogen gas which has been pre-dissolved in beer at elevated pressure will very rapidly precipitate out of solution when the beer drink flows through the dispense tap. This precipitation is in the form of a very fine dispersion of small bubbles which approaches its new lower equilibrium concentration at atmospheric pressure when the beer is dispensed.
Because these initially formed nitrogen bubbles are very small, they float slowly to the surface of the beer and some nucleate precipitation of dissolved carbon dioxide gas which enters them, causing them to grow and float faster. The small bubbles which collect at the surface thus contain nitrogen and a mixture of carbon dioxide and nitrogen gases. Because nitrogen, in comparison to carbon dioxide, is less able to permeate through the bubble wall, these bubbles are relatively stable, although they are losing carbon dioxide by permeation to the atmosphere. That loss tends to be made up by further carbon dioxide arising from the bulk of the beer in the glass. Hence the "head" on a nitrogenated beer lasts longer and is more appealing to most customers.
At the brewery or at a pub or restaurant, most beers are transported by means of pressure, generally supplied by carbon dioxide creating a high pressure of carbon dioxide in the vat or keg. Beer is transported from the vat to a bottling line or to a keg or possibly to a tap. Perhaps the most common transport of the beer for immediate presentation is from the keg to the tap. Fast transport of the beer by use of high carbon dioxide pressure, provides the risk of over carbonation of the beer. Over carbonation can lead to break out of carbon dioxide in the tubing upstream of the dispense tap when dispensing from a keg to a tap if there is a significant pressure drop in the delivery tubing. This leads to beer loss through "fobbing" i.e., production of excess foam before dispense and at the tap. In an attempt to prevent over carbonation a mixture of nitrogen and carbon dioxide gases has been used for pressure dispense of kegged beers. Although this technique helps to lessen the likelihood of over carbonation, control of a precise amount of carbonation is not feasible by this means.
It has been claimed that there is a causal relationship between the use of nitrogen in production and mixed gas in dispense. The reasoning is that if a beer has been nitrogenated initially then it should be dispensed with a mixed gas in order to maintain that nitrogenation to achieve the desired presentation effects. However, there are three implied requirements which are not independently achievable with the mixed gas dispense principle. These requirements are (1) a maximum total head pressure on the keg in order to achieve fast dispense flow rates; (2) the correct partial pressure of carbon dioxide to avoid over carbonation; and (3) the correct nitrogen partial pressure to maintain nitrogenation. No significant amount of nitrogenation of a keg beer will take place from the mixed gas pressure used for transport because at best only an equilibrium of partial pressures will be established and diffusion mobility of dissolved gases is very low in stagnant liquid layers. However, nitrogen can be lost to the head space from an initially nitrogenated beer. Commercial factors dictate in practice that the two most important requirements are a maximum total head pressure on the keg and the correct partial pressure of carbon dioxide. As a result, dispense with mixed gas is always tailored to maintaining beer carbonation and maximizing speed of dispense as opposed to maintaining the correct nitrogen content for the appeal in presentation.
Several attempts have been made to nitrogenate beer and/or control the carbon dioxide content of the beer. In French Patent application 2,684,088, a mixture of carbon dioxide and nitrogen is used as a propellant gas to transfer the beer from the keg to the tap. The pressure of the propellant gas is varied with the temperature prevailing in the stored cellar so as to ensure a constant carbon dioxide content in the beer. This method is said to maintain the optimal level of carbon dioxide in beer avoiding any desorption during transfer from the keg to the tap. However, this reference fails to deal with over carbonation or with nitrogenation.
U.S. Pat. No. 5,062,548 attempts to introduce nitrogen into the beer during the dispense operation via a restrictor at the point of dispense. All methods involving direct injection of gases at the point of dispense also have the potential drawback of sparging out desirable flavor constituents of the beer, reducing its taste and appeal. There is also a risk of bacterial growth in small orifices exposed to the beer, and effective cleaning of these orifices is difficult. This, of course, does not allow for finite control of the amount of nitrogen which may or may not dissolve in the beer, nor does it provide the very small size of bubbles of nitrogen gas which are needed to influence the presentation of the dispensed beer.
U.S. Pat. No. 5,029,733 also attempts to introduce nitrogen at the dispense point. In this case, the beverage is stored in a flexible bag and the beverage is then drawn off through a pipe and any gases desired are introduced into the beverage by way of a fixed orifice restrictor and a non-return valve. The pressure of the gas which is introduced can be adjusted; however, it can be readily seen that the control of the dissolution of nitrogen is not precise. Furthermore, the method is slow and is inconvenient.
In U.S. Pat. No. 4,610,888 a smooth stable foam is formed in a beer by intimately admixing a nitrogen containing gas, preferably air, by a venturi effect in a nozzle positioned on the beer tap. Here again some nitrogen may be introduced but the quantity is not controlled, the time to allow the nitrogen to dissolve is insufficient, and the bubbles formed are too large to affect presentation.
U.S. Pat. No. 4,364,493 provides a beverage dispensing system where the beverage is dispensed by a gaseous mixture of air and carbon dioxide. The system utilizes a diaphragm to control the admission of carbon dioxide to the container together with the air. However, this method does not assure the dissolution of nitrogen in the beer nor can it control either the carbon dioxide or nitrogen content accurately.
UK Patent application GB 2,472,225A provides a method and apparatus for dispensing gasified beverages wherein a gas permeable membrane is involved. The reference states that the beverage may be beer containing carbon dioxide in solution and where the membrane is permeable to carbon dioxide, the pressurizing gas should be a mixture of carbon dioxide and nitrogen, but where the membrane is impermeable to carbon dioxide, nitrogen may be used alone.
The present invention provides a process and apparatus to dissolve a gas in a liquid including beverages, e.g., water, soda, wine, beer and the like. With respect to beer, the present invention will (1) provide a maximum head pressure on the beer in storage to achieve fast transport flow rates; (2) provide the correct partial pressure of carbon dioxide to avoid either high or low carbonation; and (3) provide the correct partial pressure of nitrogen in the beer for a high quality presentation to the customer.